1. Field of the Invention
This invention relates to etching and, more particularly, anisotropic etching.
2. Art Background
The efficacy of a material for a particular application is often more strongly dependent on the internal geometric structure of the material than its composition. For example, the usefulness of a porous media, i.e., a body having channels or a reticulated structure, as a chemical catalyst strongly depends on the configuration of the channels or reticulations. The larger the surface area provided by a given channel or reticulation configuration generally the more efficient the catalyst.
Optical properties are also significantly affected by the internal configuration. In particular, porous bodies such as dendritic tungsten having needle-like structures with dimensions of or greater than 2 .mu.m have been employed as solar absorbers. These needle-like structures, with spacings much greater than the wavelength of visible light induce multiple reflection of light entering the area between the needles. On each reflection some absorption of light occurs and, through repeated reflections, a significant amount of light is ultimately absorbed. This enhanced absorption naturally leads to enhanced efficiency in the use of solar radiation.
Although structures such as porous bodies derive many of their attributes from their internal geometry, for some applications it has been desirable to severely limit the extent of this internal geometry. For example, electron emitters used in producing columnated electron beams for applications such as the exposure of resist materials during semiconductor device fabrication are structures that, in fact, benefit from a limited, indeed a non-existent, internal geometry. Typically, a single crystal material with a low work function, i.e., a material with a thermionic work function less than 5 eV, is formed so that it comes to a single sharp point. When an electric potential is applied, the electric field is extremely intense at this point and electron emission occurs primarily from the area of strongest field. In this manner, a relatively intense electron beam is produced.
In all the previously described situations and in a multitude of other applications, control of internal geometry is extremely important. As discussed, internal configuration is particularly significant for important applications such as those involving catalysis, optical devices, and energy transfer. Obviously, the development of methods for controlling internal structures to produce a desired configuration, and thus a desired result, is significant.